1. Technical Field
The present invention relates to an injection molding method and an injection molding machine.
2. Description of Related Art
In the past, in a stretch blow molding method that forms a container made of a resin, for example, a PET resin, a container has been formed as a finished product, that is, a finished molded product formed by forming a preform (a parison with a bottom) in advance as a preliminarily molded product, that is, a preliminary molding by an injection molding machine, setting the preform in a stretch blow molding machine, mechanically stretching the preform in an axial direction while the preform is heated and softened, and stretching the preform in a radial direction by air blowing.
Meanwhile, for example, in the case of a container that can be sealed by a screw type cap, a screw portion is formed at a spout of the container. However, the screw portion requires sufficiently high strength. For this reason, the screw portion is formed at the preform stage so as to have the same dimensions and shape as the screw portion of the container that is the finished product.
In the injection molding machine that forms a preform, like in a general injection molding machine, a resin heated and melted in a heating cylinder is filled in a cavity space of a mold device and is cooled and solidified in the cavity space, so that a preform is formed.
The injection molding machine includes the mold device, a mold clamping device, and injection device. The injection device includes the heating cylinder, an injection nozzle that is mounted on the front end of the heating cylinder and injects the melted resin, a screw that is disposed in the heating cylinder so as to be rotatable and able to freely advance and retreat, and the like. Further, the mold device includes a stationary mold and a movable mold. The movable mold is made to advance and retreat by the mold clamping device, so that the mold of the mold device is opened and closed, that is, mold closing, mold clamping, and mold opening. As the mold is clamped, a cavity space is formed between the stationary mold and the movable mold.
Further, when the screw is rotated in a metering process, the melted resin is collected on the front of the screw. Accordingly, the screw retreats, and the mold of the mold device is closed and clamped while the screw retreats. Subsequently, the screw is made to advance in the injection process, so that the resin collected on the front side of the screw is injected from the injection nozzle and is filled in the cavity space through a runner such as a hot runner formed at the mold device. Next, the resin in the cavity space is cooled and solidified in a cooling process, so that a preform is formed. Subsequently, the mold is opened and the preform is taken out.
Meanwhile, the resin is melted in the heating cylinder, and generates gas while being heated in the hot runner. Further, if the generated gas remains in the cavity space, sinks are formed on the surface of the preform or bubbles are formed in the preform. In particular, if sinks are formed on the surface of the screw portion of the preform or bubbles are formed in the screw portion, a sealing property between a cap and the screw portion of a finished container deteriorates. For this reason, it is not possible to seal the container by the cap.
Accordingly, in the mold device in the related art, a gas vent is formed at the parting lines between the stationary mold and the movable mold by a groove having a small depth, so that gas in the cavity space is discharged to the outside of the mold device through the gas vent (for example, see Patent Literature 1).